According to a programmable controller (referred to as a PLC hereinafter) used in a factory automation (referred to as an FA hereinafter), ON/OFF information is input from an input device such as a switch or a sensor, a logic operation is carried out along a sequence program (referred to as a user program also) written in ladder language or the like, and signal of the ON/OFF information is output to an output device such as a relay or a valve actuator, according to a result of the operation.
Meanwhile, the input device and the output device may be directly connected to the PLC, or may be connected thereto through a network. When a network system is constituted so as to be connected through the network, the ON/OFF information is transmitted or received through the network. At this time, the information is transmitted or received by a master/slave system normally, in which a master is on the PLC side and a slave is on the device side.
Meanwhile, a fail-safe (safety) system has been gradually introduced in the control of the PLC recently. In other words, like the PLC and the device itself, the network is constituted so as to incorporate a safety function. Here, the safety function is a function of confirming the safety and outputting it. Then, when the network system becomes in a danger state such as a case where an emergency stop switch is pressed down or a sensor such as a light curtain detects intrusion of a human (a part of a human body), the fail-safe works so that the system stops the operation on the safe side. In other words, the system generates an output to get a machine to work only when the safety is confirmed by the safety function. Therefore, when the safety is not confirmed, the machine is stopped.
In the case of the network system (safety network system) comprising the above safety function, it is necessary to keep the constant maximum response time taken from the time the abnormal and danger state is generated to the time the safety operation (stop of the apparatus or the like) is carried out. That is, as well known, when the information is transmitted by the master/slave system, each slave sequentially returns a safety response to a master according to a demand from the master as shown in FIG. 1(a). According to an example shown in FIG. 1, there are three slaves constituting the network system. The ON/OFF information used here is information for safety control such as normal (safety)/abnormal (danger). The maximum response a time is assured by time taken for one communication cycle.
Meanwhile, there is a demand for collecting complementary information (non-safety information) such as a state of the slave, an energized time and the number of operations, other than the safety information for monitoring the slave or the device connected to the slave, regularly or irregularly. When the above non-safety information is obtained, a life span of the device is determined and the device is exchanged before it is actually damaged and the system is stopped, for example.
However, as described above, when the non-safety information is transmitted, it is thought that all non-safety information is transmitted in a communication cycle 1 and all safety information is transmitted in the next communication cycle 2 in an example shown in FIG. 1(a), for example. However, according to this method, since the safety information cannot be transmitted for the communication cycle 1, the maximum response time becomes twice as long as the communication cycle in the result.
As another method, as shown in FIG. 1(b), information in which the non-safety information is added to the safety response for transmitting the safety information can be returned.
When the non-safety information is transmitted as described above, a traffic is influenced by that in both above methods, so that a communication performance of the safety information is also influenced. In other words, it should be understood that since the safety information cannot be transmitted while the non-safety information is transmitted, the transmission of the safety information is delayed for that.
Thus, since the safety slave returns the non-safety information after received the demand from the master (safety PLC), as the returning intervals are reduced, the further linear non-safety information can be monitored, but as the output intervals of the demand are shortened, an influence on the network traffic becomes large.
It is an object of the present invention to provide a safety network system and a safety slave which can effectively collect non-safety information while an influence on a network traffic is prevented as much as possible.